1. Field of the Invention
The present invention relates to a position detector, a lithography apparatus, and a device manufacturing method.
2. Description of the Related Art
A lithography apparatus such as an exposure apparatus, or the like forms a desired pattern on a substrate in a lithography process that forms part of a manufacturing processes for a semiconductor device, a liquid crystal display device, or the like. For example, the exposure apparatus transfers the pattern of the original plate (reticle or mask) onto a photosensitive substrate (such as a wafer or glass plate forming a resist layer on a surface) by use of a projection optical system. The exposure apparatus includes a position detector configured to acquire information in relation to the position of the substrate. The position detector acquires information in relation to the position of the substrate by detection of a mark formed on the substrate. For example, when an OA detection system (off-axis type alignment mark detection system) is used as the position detector, the optical axis of the OA detection system is separated from the optical axis of the projection optical system.
As a result, the alignment accuracy of a wafer relative to the projection optical system is reduced due to fluctuation in the optical axis distance (baseline) between those axes caused by environmental changes in the periphery of the OA detection system or the projection optical system. Japanese Patent Application Laid-Open No. 2000-91219 discloses a configuration in which an objective lens of the OA detection system is disposed in proximity to the optical axis of the projection optical system by use of a light path folding mirror (reflection unit). This configuration shortens the baseline between the projection optical system and the OA detection system, and thereby suppresses the reduction in the alignment accuracy by reducing the fluctuation amount in the baseline.
However, when light flux reflected from the mark on the substrate passes through the objective lens of the OA detection system in Japanese Patent Application Laid-Open No. 2000-91219, the light flux may become incident onto the light path folding mirror (reflection unit) in a convergent state or a divergent state. In this case, each light beam of the reflected light flux exhibits a different angle of incidence in relation to the reflection unit. As a result, a phase difference is produced between the light beams of the reflected light flux. Therefore, asymmetry is produced in the image that is detected by an image detection element (CCD or the like) of the OA detection system, or in the alignment mark signal resulting from photoelectric conversion of the image, and therefore, the position of the mark cannot be accurately detected.